1. Field of the Invention
The present invention relates to electrical circuits, and in particular, to transconductance cells.
2. Related Art
As is known in the art, a transconductance cell is a basic electrical circuit or block used to build more complex electrical circuits, such as low noise amplifiers and analog filters. The transconductance cell performs the function of converting a voltage input into different current outputs, such as by varying the transconductance gm of the cell (iout=gm*vin) The characteristics of a desirable transconductance cell include high bandwidth, low power consumption, high output impedance, low distortion, and good common mode rejection. Furthermore, with an ever-increasing need and use of high speed analog circuits and chips, transconductance cells should be able to provide these characteristics at high speeds with wide linear dynamic range and low power dissipation.
FIG. 1A shows a conventional transconductance cell 100, in which transconductance gm is varied by varying the current. Transconductance cell 100 includes two transistors 102 and 104, such as N-channel MOS transistors, resistive or impedance load elements 106 and 108 connected between the drain of transistors 102 and 104, respectively, and a voltage source 110, and a variable current source 112 connected between the source of both transistors 102 and 104 and ground. The transconductance is varied in such a cell by varying the amount of bias current generated by current source 112, such as with a control signal, of the differential transconductance pair. This, however, changes the linearity and increases power dissipation. Furthermore, to increase the gain (where gain is equal to gm*RL (the load resistance)) by m, the drain current ID needs to be increased by a factor of m2. The large increase in the drain current results in a large overhead in power dissipation. The voltage headroom (Vds, viz. drain to source voltage of a MOS transistor) is also lowered and the variation in linearity is disadvantageously widened.
FIG. 1B shows another conventional transconductance cell 140, in which gain is changed by varying the load resistance. The structure of cell 140 is the same as cell 100 of FIG. 1A, except that load elements 106 and 108 are variable and current source 112 is constant. The load resistance of load elements 106 and 108 can be changed by varying characteristics of the components forming load elements 106. For example, load elements 106 may include an inductor and resistor in series (for a load impedance). The load impedance can then be changed by varying the resistance of the resistor and/or the inductance of the inductor. However, such a transconductance cell has limited gain controllability at higher speeds, e.g., in the multi-GHz range. Further, if the gain is to be increased, e.g., by a factor of m, the load resistance RL must be increased by m. This reduces the bandwidth BW of the device by m, since BW is proportional to 1/RL (more specifically, BW=1/(2πRLCL), where CL is the load capacitance).
FIG. 1C shows a third kind of transconductance cell 180 that uses source degeneration to maintain a constant transconductance gmfor the device. Cell 180 includes two transistors 102 and 104 coupled together at the respective sources by two resistors 182 and 184 in series. Current sources 186 and 188 are coupled to the respective sources of transistors 102 and 104. When the gate voltage is changed, the saturation current changes, with some of the current flowing through the resistors. This causes the source voltage to increase, which reduces the original increase in the saturation current caused by the increase in the gate voltage. The transconductance is reduced from its value with the source voltage held constant. Mathematically, the effective gm for this structure can be shown to be as follows:       g          m      eff        =            g      m              1      +                        g          m                ⁢                  R          s                    where Rs is the source degeneration resistance associated with resistors 182 and 184, which is varied to get variable transconductance. Hence, in this type of cell, the resistances associated with resistors 182 and 184 can be shown to be varying. However, such cells 180 can only be used at low speeds, since the effective lowering of the inherent gm reduces the transit frequency (Ft) of the device.
Accordingly, there is a need for a transconductance cell that provides variable transconductance at low power dissipation, while maintaining high bandwidth and linearity.